When Daisy first sees Gatsby's mansion from Nick's front yard, she is shocked, asking, "'That huge place there?'" She says that she loves it, but it is so gigantic that she is surprised that he could possibly live in it all by himself. She admires the gardens, the lovely scents of the flowers, the picture of the house against the blue sky. As Daisy walks around Gatsby's mansion, she seems to appreciate every detail. In his bedroom, she notices a toiletry set composed of gold, and she sits down to brush her hair "with delight." He begins to take out his beautiful, imported shirts, and throw them, one after the other, at Nick and Daisy, when "Suddenly, with a strained sound, Daisy bent her head into the shirts and began to cry stormily." She says that she is sad because she's never seen such lovely shirts, although this explanation does seem unlikely. Perhaps she cries because she realizes that, if she'd just been a bit more patient, she could have not married Tom, waited for Gatsby, and not have sacrificed her financial wealth. Perhaps she cries because she realizes that it's too late for them to marry. Nick never knows.
I assume that you are talking about the part in Chapter 5 when Nick arranges the meeting between Gatsby and Daisy.
Gatsby has asked Nick to do this so that he can (hopefully) get back together with Daisy. At their meeting, Daisy and Gatsby start to rekindle their old spark. Then Gatsby has Daisy come over to look at his house. She is really impressed and she is overcome with emotion. She starts crying as she looks at his shirts.
Some people say that this is because she realizes that he has become rich and that she would have been happy if she had married him. But now it's too late and she can't marry him.